docs: def _destinations_in_two_columns(pdf, destinations, cutoff=3):
      """
      Check if the named destinations are organized along two columns (heuristic)
  
      @param pdf: a PdfFileReader object
      @param destinations:
  
      'cutoff' is used to tune the heuristic: if 'cutoff' destinations in the
      would-be second column start at the same position, return True
      """
      # iterator for the x coordinates of refs in the would-be second column
      xpositions = (_destination_position(pdf, dest)[3] for (_, dest)
                    in destinations
                    if _destination_position(pdf, dest)[1] == 1)
      xpos_count = {}
      for xpos in xpositions:
          xpos_count[xpos] = xpos_count.get(xpos, 0) + 1
          if xpos_count[xpos] >= cutoff:
              return True
      return False